A diversity receiver is adapted to receive an electromagnetic wave in a good condition transmitted from a signal source which transmits the wave while moving, by reducing effects of dead points. An example of such signal source is a wireless microphone. There are various types of diversity system. An antenna diversity system is expected to be the most cost effective system. This system employs a plurality of receiving antennas and a single receiving unit. The receiving antennas are disposed at different locations in a communication area. One of signals received at the antennas is coupled to the receiving unit through an antenna switching unit. The antenna switching unit is operated in such a manner that the receiving level at the receiving unit can be larger.
If the signal source transmits an analog signal, an instantaneous value detected at an analog receiver receiving such analog signal constitutes part of the demodulation output. However, if the signal source is digitally transmitting, an instantaneous value received by a digital receiver receiving such a digital signal can form only part of digital data forming a digital data train. A digital data train includes plural pieces of digital data, and each digital data piece is a quantized version of each sampling value produced by sampling an audio or video signal at a predetermined sampling frequency. In order to produce a sound or a picture ultimately in a digital receiver, part of digital data detected from time to time must be collected for each sample in an appropriate arrangement. Therefore, it is necessary to synchronize data pieces for respective samples. If synchronization is not established, the demodulated audio output or video output cannot be assured to correspond to the digital data train, but queer sound would appear in the reproduced sound output, or a picture different from the original one would be reproduced.
In a digital wireless receiver employing an antenna diversity system as described above, each time the antenna is switched, it is necessary to establish synchronization for respective samples on the basis of a signal received at a new antenna selected by the switching. In a cellular phone and digital cordless phone, which are digital wireless systems, but employ time-division multiplexing system, the digital wireless receiver needs to synchronize only when it is receiving a receiver channel assigned to it. (According to the time-division multiplexing, a single high-frequency channel is temporally divided, and channels are assigned to various receivers.) Accordingly, the antenna switching can be done when channels for other receivers are being received.
On the other hand, in a unidirectional communication system, which is not a time-division multiplexing system, and in which a set of transmitter and receiver exclusively uses one high-frequency channel, e.g. a system including a wireless microphone and a dedicated receiver therefor, the dedicated receiver receives the one high-frequency channel all the time, and, therefore, each time a different antenna is selected, it must be re-synchronized.
An object of the present invention is to make it possible to smoothly re-synchronize an antenna diversity receiver for use in a unidirectional digital communication.